What Is a Case-Control Study? | Definition & Examples

A case-control study is an experimental design that compares a group of participants possessing a condition of interest to a very similar group lacking that condition. Here, the participants possessing the attribute of study, such as a disease, are called the “case,” and those without it are the “control.”

It’s important to remember that the case group is chosen because they already possess the attribute of interest. The point of the control group is to facilitate investigation, e.g., studying whether the case group systematically exhibits that attribute more than the control group does.

Example: Case-control study
You are interested in the effects of exposure to a particular chemical on diagnoses of mesothelioma, a type of cancer. Here, you would compare those exposed (your case group) with those not exposed (your control group) to the chemical in question.

This would allow you to observe whether the people exposed to the chemical had more instances of mesothelioma than those who weren’t exposed.

When to use a case-control study

Case-control studies are a type of observational study often used in fields like medical research, environmental health, or epidemiology. While most observational studies are qualitative in nature, case-control studies can also be quantitative, and they often are in healthcare settings. Case-control studies can be used for both exploratory and explanatory research, and they are a good choice for studying research topics like disease exposure and health outcomes.

A case-control study may be a good fit for your research if it meets the following criteria.

  1. Data on exposure (e.g., to a chemical or a pesticide) are difficult to obtain or expensive.
  2. The disease associated with the exposure you’re studying has a long incubation period or is rare or under-studied (e.g., AIDS in the early 1980s).
  3. The population you are studying is difficult to contact for follow-up questions (e.g., asylum seekers).
Tip
Many students confuse case-control studies with cohort studies, particularly retrospective cohort studies. While they may seem similar (and are indeed both types of retrospective observational studies), they follow distinct research paths.

Retrospective cohort studies use existing secondary research data, such as medical records or databases, to identify a group of people with a common exposure or risk factor and to observe their outcomes over time. Case-control studies conduct primary research, comparing a group of participants possessing a condition of interest to a very similar group lacking that condition in real time.

Additionally, cohort studies in general are more longitudinal in nature and do not necessarily require a control group. While one may be added if the investigator so chooses, members of the cohort are primarily selected because of a shared characteristic among them.

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Examples of case-control studies

Case-control studies are common in fields like epidemiology, healthcare, and psychology.

Example: Epidemiology case-control study
You are examining the relationship between drinking water contamination and the incidence of gastrointestinal illnesses like gastroenteritis. Here, the case group would be individuals who have been diagnosed with a gastrointestinal illness, while the control group would be individuals without such an illness.

You would then collect data on your participants’ exposure to contaminated drinking water, focusing on variables such as the source of said water and the duration of exposure, for both groups. You could then compare the two to determine if there is a relationship between drinking water contamination and the risk of developing a gastrointestinal illness.

Example: Healthcare case-control study
You are interested in the relationship between the dietary intake of a particular vitamin (e.g., vitamin D) and the risk of developing osteoporosis later in life. Here, the case group would be individuals who have been diagnosed with osteoporosis, while the control group would be individuals without osteoporosis.

You would then collect information on dietary intake of vitamin D for both the cases and controls and compare the two groups to determine if there is a relationship between vitamin D intake and the risk of developing osteoporosis.

Example: Psychology case-control study
You are studying the relationship between early-childhood stress and the likelihood of later developing post-traumatic stress disorder (PTSD). Here, the case group would be individuals who have been diagnosed with PTSD, while the control group would be individuals without PTSD.

You would then collect information on any history of early life stress (e.g., abuse, neglect, trauma) for both the cases and controls and compare the two groups to determine if there is a relationship between early life stress and the risk of developing PTSD.

Advantages and disadvantages of case-control studies

Case-control studies are a solid research method choice, but they come with distinct advantages and disadvantages.

Advantages of case-control studies

  • Case-control studies are a great choice if you have any ethical considerations about your participants that could preclude you from using a traditional experimental design.
  • Case-control studies are time efficient and fairly inexpensive to conduct because they require fewer subjects than other research methods.
  • If there were multiple exposures leading to a single outcome, case-control studies can incorporate that. As such, they truly shine when used to study rare outcomes or outbreaks of a particular disease.

Disadvantages of case-control studies

  • Case-control studies, similarly to observational studies, run a high risk of research biases. They are particularly susceptible to observer bias, recall bias, and interviewer bias.
  • In the case of very rare exposures of the outcome studied, attempting to conduct a case-control study can be very time consuming and inefficient.
  • Case-control studies in general have low internal validity and are not always credible.

Case-control studies by design focus on one singular outcome. This makes them very rigid and not generalizable, as no extrapolation can be made about other outcomes like risk recurrence or future exposure threat. This leads to less satisfying results than other methodological choices.

Other interesting articles

If you want to know more about statistics, methodology, or research bias, make sure to check out some of our other articles with explanations and examples.

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Frequently asked questions

What’s the difference between a case-control study and a cohort study?

A case-control study differs from a cohort study because cohort studies are more longitudinal in nature and do not necessarily require a control group.

While one may be added if the investigator so chooses, members of the cohort are primarily selected because of a shared characteristic among them. In particular, retrospective cohort studies are designed to follow a group of people with a common exposure or risk factor over time and observe their outcomes.

Case-control studies, in contrast, require both a case group and a control group, as suggested by their name, and usually are used to identify risk factors for a disease by comparing cases and controls.

What’s the difference between a case-control study and a cross-sectional study?

A case-control study differs from a cross-sectional study because case-control studies are naturally retrospective in nature, looking backward in time to identify exposures that may have occurred before the development of the disease.

On the other hand, cross-sectional studies collect data on a population at a single point in time. The goal here is to describe the characteristics of the population, such as their age, gender identity, or health status, and understand the distribution and relationships of these characteristics.

How are cases and controls selected in a case-control study?

Cases and controls are selected for a case-control study based on their inherent characteristics. Participants already possessing the condition of interest form the “case,” while those without form the “control.”

Keep in mind that by definition the case group is chosen because they already possess the attribute of interest. The point of the control group is to facilitate investigation, e.g., studying whether the case group systematically exhibits that attribute more than the control group does.

How can the strength of association between an exposure and a disease be measured in a case-control study?

The strength of the association between an exposure and a disease in a case-control study can be measured using a few different statistical measures, such as odds ratios (ORs) and relative risk (RR).

Can case-control studies establish causality?

No, case-control studies cannot establish causality as a standalone measure.

As observational studies, they can suggest associations between an exposure and a disease, but they cannot prove without a doubt that the exposure causes the disease. In particular, issues arising from timing, research biases like recall bias, and the selection of variables lead to low internal validity and the inability to determine causality.

Sources in this article

We strongly encourage students to use sources in their work. You can cite our article (APA Style) or take a deep dive into the articles below.

This Scribbr article

George, T. (2023, June 22). What Is a Case-Control Study? | Definition & Examples. Scribbr. Retrieved June 11, 2024, from https://www.scribbr.com/methodology/case-control-study/

Sources

Schlesselman, J. J. (1982). Case-Control Studies: Design, Conduct, Analysis (Monographs in Epidemiology and Biostatistics, 2) (Illustrated). Oxford University Press.

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Tegan George

Tegan is an American based in Amsterdam, with master's degrees in political science and education administration. While she is definitely a political scientist at heart, her experience working at universities led to a passion for making social science topics more approachable and exciting to students.